fix: Use HasField for simple message retrieval for grpc transport

Using `response.task` will reset the oneof field if the `msg` field was
set. Properly check and fix return value.

Author: timn

src/a2a/client/transports/grpc.py

@@ -87,7 +87,7 @@ async def send_message(
                 metadata=proto_utils.ToProto.metadata(request.metadata),
             )
         )
-        if response.task:
+        if response.HasField('task'):
             return proto_utils.FromProto.task(response.task)
         return proto_utils.FromProto.message(response.msg)